This invention relates to monopulse radars, and in particular to means for effecting simultaneous nulling in the sum and difference patterns of a monopulse phased array antenna using one set of phase shifters shared by both channels.
In the past few years, considerable research and development has been expended on adaptive antennas. Communications and sonar systems have reaped some of the benefits of adaptive technology while radars generally have lagged behind. Some of the reasons for this dichotomy are that many adaptive techniques are not suited for microwave frequencies; radars have large antennas, hence more adaptive loops; and a radar has tight time constraints for detecting and tracking targets. Consequently, only a handful of radars incorporating sidelobe cancelling techniques exist today.
Monopulse radars add additional problems to the adaptive processing. Monopulse tracking radars use a multiple beam antenna to determine the position of a target. The sum far-field antenna pattern has its peak in the direction of the target, while a difference pattern has its null on the target. By taking the ratio of the difference channel output to the sum channel output, enough information is obtained to track the target accurately. Assuming the target is on boresite, the sum channel has a strong output signal and the difference channel has very little output since the target is in the null. The resulting monopulse ratio is zero. When the target moves, it moves out of the difference pattern null into either the positive lobe or a negative lobe of the difference pattern. In turn, the monopulse ratio changes to a small positive or negative value. The target angular position information is then deducted from the monopulse ratio.
Interference from the environment or other electromagnetic systems can seriously degrade the tracking performance of a monopulse radar. An interference in the sum channel will destroy range and resolution information as well as tracking data. The undesired signals enter the sidelobes of both the sum and difference far-field patterns and add with the desired signal in the main beam direction. If the interference is strong enough, the desired signal will not be detected. As a result, the radar's detection range is decreased to a point where the return signal from a target can be detected. Consider a simple form of the radar range equation given by: ##EQU1## where R.sub.max =maximum radar range
P.sub.t =transmitted power PA0 G=transmitting antenna gain PA0 A.sub.e =receiving antenna effective aperture (area) PA0 .sigma.=target radar cross section PA0 S.sub.min =minimum detectable signal (power)
In the presence of interference, S.sub.min will increase because the desired signal must now be greater than the interference signal, rather than just the receiver noise (assuming the interference signal&gt;receiver noise). Thus, the interfering signal poses a serious threat to radar systems by limiting the maximum range of the radar.
Interference reduction techniques are necessary to improve antenna performance in a heavy signal environment. Of particular interest are fully adaptive phased array antennas. An adaptive antenna has the ability to place nulls in its far-field radiation pattern in the direction of interference sources. The depths and bandwidths of the nulls determine how well interference sources are suppressed. These nulls are placed in the far-field pattern by adaptively adjusting the phases of the signals at the elements in a phased array.
Putting a null in the sum far-field pattern (in the direction of interference) will not necessarily result in a similar null in the far-field difference pattern. Consequently, the detection capability will improve, but tracking capability, in general, will not. In most situations, it would be desirable to improve the radar's detection and tracking performance at the same time. This can be done by simultaneous adaptive nulling in the sum and difference channels. Either the sum and difference channels must be adapted separately, or a technique of simultaneously nulling in both channels must be used.
In a copending application by the present inventor entitled "Simultaneous Nulling In The Sum The Difference Patterns of a Monopulse Antenna", having Ser. No. 498,334, and filed May 26, 1983, there is disclosed apparatus and a technique for simultaneously placing nulls in the sum and difference patterns of a monopulse phased array antenna. A preferred embodiment of this earlier invention includes circuitry for not only shifting the phase of the signals, but includes means for also adjusting the amplitude of the signals.
A significant shortcoming of this prior invention however, is that the location of the jammer sources must be known. A further limitation is that the phase only implementation thereof is limited to use in very low sidelobe antennas. Also the depths of the nulls achieved by this prior invention are limited.